Long Term Evolution (LTE) wireless networks are being standardized by the 3rd Generation Partnership Project (3GPP) working groups. The general operations of the physical channels are described in the LTE specifications, for example: “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (3GPP TS 36.211 Release 8).”
LTE networks may incorporate a number of user equipments (UEs) and a number of Node Bs. A UE, also commonly referred to as a terminal or a mobile station, may be a wired or wireless device, such as a cellular phone, a notebook computer, a personal digital assistant (PDA), a wireless modem card, and so on. A Node B may be a fixed station, such as a base transceiver system (BTS), an access point (AP), a base station (BS), etc. As improvements of networks are made, the Node B may evolve to provide different functionalities, so a Node B may also be referred to as an evolved Node B (eNB).
An eNB may contain radio frequency transmitter(s) and receiver(s) to communicate with a UE. Similarly, a UE may contain radio frequency transmitter(s) and receiver(s) to communicate with an eNB. Communication from the UE to the eNB may be referred to as Uplink communication (UL), whereas communication from the eNB to the UE may be referred to as downlink (DL) communication. Multiple UEs may be time and frequency multiplexed onto a UL or a DL channel. The UL channel may include a physical uplink shared channel (PUSCH), and a physical uplink control channel (PUCCH). The DL channel may include a physical downlink shared channel (PDSCH) and a physical downlink control channel (PDCCH).
A UE may estimate channel conditions and then report the estimated channel conditions to the eNB in order to gain better performance out of the channel. The process of UE providing the channel condition information to the eNB may be referred to as channel state information (CSI) feedback. The CSI feedback may include a channel quality index (CQI), a precoding (beamforming) matrix indicator (PMI), and a rank indication (RI), which are various matrices that may be measured or otherwise derived at the UE. As the channel may be continuously changing, the UE may provide the CSI feedback to the eNB for multiple points across the channel bandwidth, at regular time intervals, or aperiodically, up to several hundred times a second.
PMI may reflect the recommended precoding matrices within the recommended rank. PMI may uniquely identify a precoding matrix (precoder) within a codebook shared by the UE and the eNB. The accuracy of PMI feedback may be related to the size of the codebook. In general, the bigger the codebook, the smaller the quantization error may be when the precoding matrix is estimated by the UE. However, in practice, the size of the codebook may not be increased arbitrarily for various reasons, including, for example, to maintain backward compatibility, and to reduce feedback overhead.
It may be possible to reduce quantization error in the CSI feedback by reporting multiple PMIs. For example, proposals to the draft 3GPP Technical Specification (TS) 36.213, Release 10 include suggestions to improve the wideband performance by reporting multiple subband PMIs via PUSCH under mode 3-2. Similar suggestions have also been proposed to improve the feedback accuracy in other reporting modes, including PUCCH under mode 2-1, (reporting a subband PMI in addition to a wideband PMI), and PUSCH under mode 3-1, (reporting multiple wideband PMIs).
The gain of sending multiple subband PMIs compared with sending only one wideband PMI may depend on the frequency selectivity of the channel, which may be related to 1) antenna spacing and configuration; 2) inter-site distance and 3) random delay error in the transmission antennas. When the channel is not highly frequency selective, sending more subband PMIs may not bring high gains because the subband PMIs may be similar to the wideband PMI. On the other hand, when the channel is highly frequency selective, sending more subband PMIs may improve the wideband performance significantly. As such, there may be a trade-off between feedback overhead and beamforming performance.